816 Transactions of the Society. 



XVII. — On " Optical Tuhe-length " ; an Unconsidered Element 

 in the Theory of the Microscope. 



By Frank Crisp, V.P.L.S., Sec. K.M.S. 



{nead lAth November, 1883.) 



It is not a little strange that at this late period in the develop- 

 ment of the Microscope, an element of capital importance both from 

 a theoretical and a practical point of view should have been left 

 entirely unconsidered, and indeed unknown ; and the fact that it is 

 so, illustrates the disadvantages which English-speaking micro- 

 scopists have always been under in having no text-book dealing 

 with the theory of the Microscope. 



In a letter written more than a year ago in reference to the 

 Table of Magnifying Powers published in the Journal, Professor 

 Abbe called my attention to the erroneous notions which prevailed 

 on the subject of the magnifying power of the Microscope, and 

 which he had been the first to clear up,* and I ought then to have 

 published the explanation now given here, but the pressure of other 

 engagements diverted my attention, and 1 confined myself to ex- 

 plaining the matter verbally to those who attended the meetings. 

 Finding, however, that the Committee on Eye-pieces of the 

 American Society of Microscopists have been misled by the Table 

 in question, it is obviously desirable not to delay the explanation 

 any longer. 



Microscopists have always recognized that the length of the 

 tube of the Microscope is a factor in determining the amplification 

 of the image, that the amplification is generally greater with a 

 10 in. tube than with one of 6 in. ; and that we obtain an increase 

 of power by pulling out the draw-tube. Here, however, all exact 

 notions as to the function of the tube-length have practically 

 stopped, so much so that there has not been any agreement even as 

 to how the length of the tube is to be measured, whether from the 

 front or back lens of the objective to the field lens, the diaphragm, 

 or the eye lens of the eye-piece. 



In particular, no view of tube-length has been held which would 

 explain the following apparently paradoxical statements : — 



That two objectives of precisely the same focal length used with 

 the same tube and the same eye-piece may nevertheless give 

 different magnifying powers. 



That two objectives of different focal lengths used with the same 



* Professor Abbe also communicated it to Dr. Dippel, by whom it was 

 embodied in the hist edition of ' Das Mikroskop,' 1882. 



